feat: add per-IP rate limiting to login endpoint (#78)

## Summary

Adds per-IP rate limiting to `POST /api/v1/login` to prevent brute-force password attacks.

closes #35

## What Changed

### New package: `internal/ratelimit/`

A generic per-key token-bucket rate limiter built on `golang.org/x/time/rate`:
- `New(ratePerSec, burst)` creates a limiter with automatic background cleanup of stale entries
- `Allow(key)` checks if a request from the given key should be permitted
- `Stop()` terminates the background sweep goroutine
- Stale entries (unused for 15 minutes) are pruned every 10 minutes

### Login handler integration

The login handler (`internal/handlers/auth.go`) now:
1. Extracts the client IP from `X-Forwarded-For`, `X-Real-IP`, or `RemoteAddr`
2. Checks the per-IP rate limiter before processing the login
3. Returns **429 Too Many Requests** with a `Retry-After: 1` header when the limit is exceeded

### Configuration

Two new environment variables (via Viper):

| Variable | Default | Description |
|---|---|---|
| `LOGIN_RATE_LIMIT` | `1` | Allowed login attempts per second per IP |
| `LOGIN_RATE_BURST` | `5` | Maximum burst of login attempts per IP |

### Scope

Per [sneak's instruction](#35), only the login endpoint is rate-limited. Session creation and registration use hashcash proof-of-work instead.

## Tests

- 6 unit tests for the `ratelimit` package (constructor, burst, burst exceeded, key isolation, key tracking, stop)
- 2 integration tests in `api_test.go`:
  - `TestLoginRateLimitExceeded`: exhausts burst with rapid requests, verifies 429 response and `Retry-After` header
  - `TestLoginRateLimitAllowsNormalUse`: verifies normal login still works

## README

- Added "Login Rate Limiting" subsection under "Rate Limiting & Abuse Prevention"
- Added `LOGIN_RATE_LIMIT` and `LOGIN_RATE_BURST` to the Configuration table

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #78
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/api_test.go

@@ -2388,6 +2388,109 @@ func TestSessionUsernameDefault(t *testing.T) {
 	_ = token
 }
 
+func TestLoginRateLimitExceeded(t *testing.T) {
+	tserver := newTestServer(t)
+
+	// Exhaust the burst (default: 5 per IP) using
+	// nonexistent users. These fail fast (no bcrypt),
+	// preventing token replenishment between requests.
+	for range 5 {
+		loginBody, mErr := json.Marshal(
+			map[string]string{
+				"nick":     "nosuchuser",
+				"password": "doesnotmatter",
+			},
+		)
+		if mErr != nil {
+			t.Fatal(mErr)
+		}
+
+		loginResp, rErr := doRequest(
+			t,
+			http.MethodPost,
+			tserver.url("/api/v1/login"),
+			bytes.NewReader(loginBody),
+		)
+		if rErr != nil {
+			t.Fatal(rErr)
+		}
+
+		_ = loginResp.Body.Close()
+	}
+
+	// The next request should be rate-limited.
+	loginBody, err := json.Marshal(map[string]string{
+		"nick": "nosuchuser", "password": "doesnotmatter",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	resp, err := doRequest(
+		t,
+		http.MethodPost,
+		tserver.url("/api/v1/login"),
+		bytes.NewReader(loginBody),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusTooManyRequests {
+		t.Fatalf(
+			"expected 429, got %d",
+			resp.StatusCode,
+		)
+	}
+
+	retryAfter := resp.Header.Get("Retry-After")
+	if retryAfter == "" {
+		t.Fatal("expected Retry-After header")
+	}
+}
+
+func TestLoginRateLimitAllowsNormalUse(t *testing.T) {
+	tserver := newTestServer(t)
+
+	// Create session and set password via PASS command.
+	token := tserver.createSession("normaluser")
+
+	tserver.sendCommand(token, map[string]any{
+		commandKey: "PASS",
+		bodyKey:    []string{"password123"},
+	})
+
+	// A single login should succeed without rate limiting.
+	loginBody, err := json.Marshal(map[string]string{
+		"nick": "normaluser", "password": "password123",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	resp2, err := doRequest(
+		t,
+		http.MethodPost,
+		tserver.url("/api/v1/login"),
+		bytes.NewReader(loginBody),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = resp2.Body.Close() }()
+
+	if resp2.StatusCode != http.StatusOK {
+		respBody, _ := io.ReadAll(resp2.Body)
+		t.Fatalf(
+			"expected 200, got %d: %s",
+			resp2.StatusCode, respBody,
+		)
+	}
+}
+
 func TestNickBroadcastToChannels(t *testing.T) {
 	tserver := newTestServer(t)
 	aliceToken := tserver.createSession("nick_a")